The present invention relates to phase gratings for diffracting light beams by phase control, and more particularly, to phase gratings which have combined characteristics of a pattern modification type and a refraction modification type phase grating.
In general, a phase change P which is caused when light beams of wavelength .lambda. travel a distance D within a light medium of a refractive index n is shown by the equation ##EQU1## When a periodic distribution of the phase variation P is caused on an exit plane of the light medium due to variations in the distance D and the refractive index n, the light beams are diffracted in such a direction that light beam components with different phase changes P amplify each other by phase matching.
Conventional phase gratings are broadly divided into the following two categories. One, as shown in FIG. 3, is a pattern modification type phase grating 1A which comprises a transparent plate 2A including regularly arranged convex-concave portions a, b for controlling phases of light beams and the other, as shown in FIG. 4, is a refraction modification type phase grating 1B which comprises a transparent plate 2B including regularly arranged layers a', b' of different refractive indexes, respectively, for controlling phases of light beams which pass therethrough.
The phase grating 1A shown in FIG. 3 is in common use for light branching elements, diffraction gratings for dividing light beams, Fresnel lenses and the like. With the phase grating 1A, however, in order to amplify diffracted light beams other than zero-order diffracted light beams by suppressing the latter, it is necessary to increase the difference in level of regularly arranged convex-concave portions, so that a high degree precision working is required when the diffraction angle is increased by minimizing a pitch of the convex-concave portions, resulting in difficulty in manufacturing phase gratings 1A.
In the phase grating 1B shown in FIG. 4, a method for amplifying diffracted light beams other than zero-order diffracted light beams by suppressing the latter is to increase the difference in refractive index between regularly arranged layers or thickness of the grating 1B. However, the increasable range of difference in refractive index is comparatively limited in practice, so that the phase grating 1B is disadvantageous in that its thickness is extremely increased as compared with the pattern modification type phase grating 1A.